An ongoing environmental challenge is managing the legacy of anthropogenic activities that have resulted in the contamination of groundwater, surface water, soil, and/or soil gas via subsurface source zones. Source zones may be defined as saturated or unsaturated subsurface regions containing hazardous substances, pollutants, or reactive materials that may act as reservoirs that sustain a reactive material plume in groundwater, surface water, or air or may act as sources for direct exposure. These source zones may include sorbed and aqueous-phase reactive materials as well as non-aqueous reactive materials such as solids or nonaqueous phase liquids (NAPLs).
The identification and cleanup of source zones may be difficult due to heterogeneity within the subsurface formations in the form of spatial variations in permeability and porosity that may result in the sparse distribution of the reactive materials within the subsurface formations. As a result, locating regions within source zones that require treatment is difficult. Furthermore, monitoring the reactive materials within source zones and the effect of remedial agents on reactive materials within source zones is challenging due to the relative inaccessibility of these subsurface formations, which may extend well over about 30 feet below ground surface.
The monitoring of reactive materials introduced into preexisting subsurface formations poses an ongoing challenge for the environmental management of various industrial facilities as well as waste collection, processing, and storage facilities. Existing monitoring methods may exploit the microbially-mediated biodegradation of organic reactive materials, which may produce carbon dioxide and heat within a contaminated region of the subsurface formation. These existing methods, such as the Licor Trap Method or the CO2 Trap Method, estimate reactive material loss rates using measured efflux of CO2 above contaminated subsurface formations, assuming that the amount of CO2 measured is directly related to the reactive material loss rate resulting from biodegradation. These CO2-based methods provide estimates based on a small area over a short period of time, but are less effective at providing the long-term monitoring capability needed to assess ongoing contamination events or to assess the effectiveness of remediation of the contaminated subsurface formation. Chimneying, ground surface cover, heterogeneities in the soil, and short or long term climatic events may cause a large variability in reactive material loss rates estimated using these existing methods. Further, inorganic reactive materials, such as aluminum, may not be compatible with these existing methods because the degradation reactions within the contaminated subsurface formations may not produce CO2.
A need exists for a robust method for monitoring the rate of change of an amount of reactive material within a subsurface formation using measurements that are compatible with a wide variety of potential reactive materials. Such a method may be used to monitor the degree of contamination of a subsurface formation, to detect the introduction of additional reactive materials into the formation, to assess the rate of degradation of the reactive materials, and to assess the effectiveness of remediation of the contaminated subsurface formation.